Object lens unit and optical pickup device employing the unit

ABSTRACT

An object lens unit for an optical pickup comprising: a first optical element positioned facing an optical information recording medium; a second optical element positioned on a light source side of the first optical element; and a frame which has a first reference surface in contact with the first optical element to position the first optical element in an optical axis direction and a second reference surface in contact with the second optical element to position the second optical element in the optical axis direction so that the first optical element and the second optical element are fixed to be integrated with the frame, wherein the first optical element, the second optical element and the frame are attached, as a single optical component, to another member.

This application is based on Japanese Patent Application No. 2004-270233filed on Sep. 16, 2004 in Japanese Patent Office, the entire content ofwhich is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to optical pickup devices for recordingand reproducing optical information recording media, and to an objectlens unit used in such optical pickup devices.

In recent years, with the practical application of short wavelength redsemiconductor lasers, the Digital Versatile Disk (DVD) which is a highdensity optical disk having a large memory capacity and being of aboutthe same size as a CD (Compact Disk), which is a conventional opticaldisk (also called an optical information recording medium) has beendeveloped and commercialized.

Following this, optical pickup devices have been developed that arecompatible with several types of optical information recording media(specifically, CDs and DVDs). As a pickup of this type, known is one(for example, see Patent Document 1) that features a reduced size and issimpler by using lasers with different wavelengths depending on the typeof optical information recording medium, forming diffracted light usingdiffraction optical elements, and focusing the diffracted light ofdifferent orders as information recording light or informationreproducing light depending on the wavelength by using a condenser lens.In addition, a phase control element (diffraction optical element) andan object lens (condenser lens) are fixed by a holder within the pickupdevice, and in particular, a type has been disclosed (see PatentDocument 2) in which both the phase control element and the object lenshave respective positioning marks to adjust the central axes.

On the other hand, in the near future, still next-generation opticaldisks of higher density are expected to be commercialized. In thecondensing optical systems of optical pickup devices using suchnext-generation optical disks as a medium, in order to achieve highdensity of recording signals, or in order to reproduce high-densityrecorded signals, it is required to reduce the diameter of the focusedspot on the information recording surface using an object lens. Thisrequires shorter laser wavelengths, which is a light source, and ahigher numerical aperture number of the object lens. A blue-violetsemiconductor laser of a wavelength of approximately 400 nm is the laserthat is expected to soon be practically applied as a short wavelengthlaser light source.

As an example of the research and development of high-density opticaldisk system that can carry out recording and reproduction of informationusing such a short wavelength laser light source, well-known ones are anoptical disk that can carry out information recording and reproductionwith a specification of an NA (Numerical Aperture) of 0.85 and a lightsource wavelength of 405 nm (such as the Blu-ray Disk), and an opticaldisk that can carry out recording and reproduction of information at aspecification of an NA of 0.65 and a light source wavelength of 405 nm(for example, the HDDVD). (Hereinafter, all optical disks with aspecification of a light source wavelength of 405 nm are collectivelyreferred to as “High-Density DVD”.)

Following the development of such high-density DVDs, a lens unit hasbeen proposed for an optical pickup unit that integrates a diffractingoptical element and a focusing lens and that carries out recording andreproduction of information from three types of optical informationrecording media conforming to different standards such as high-densityDVD, normal DVD, and CD (see Patent Document 3).

-   -   Patent Document 1: Tokkai No. 2001-93179    -   Patent Document 2: Tokkai No. 2001-6203    -   Patent Document 3: Tokkai No. 2004-62971

However, in the technologies disclosed in Patent Documents 1 and 2,there still remain the problems of alignment of various optical elementsthat are the constituent elements of the optical system in themanufacturing of condensing optical system for optical pickups. Forexample, in Patent Document 1, although it has been described that thecondensing lens and the diffraction optical element are placed coaxiallywith the optical axis by a holder, a specific clear description has notbeen given regarding the positioning in the direction of the opticalaxis nor about any concrete method for the coaxial placement. Inaddition, in Patent Document 2, although there is description ofalignment using positioning marks on the phase control element(diffraction optical element) and on the object lens (the condensinglens), there is no description of the positioning related to thedirection of the optical axis. Further, in both Patent Documents 1 and2, each of the optical elements is mounted directly as a separatecomponent onto the holder which is a part of the optical pick-up device.In other words, the accuracy of relative positioning between thedifferent optical elements and the accuracy of alignment of each opticalelement based on the holder is extremely important in order to increasethe image forming performance in the condensing optical system. However,in the above technologies, the work becomes complicated because the workof relative positioning between the different optical elements and thework of their alignment relative to the holder are incorporated in theassembly process of the optical pickup device, whereby it is difficultto obtain high accuracy of condensing or image formation by a condensingoptical system comprising two optical elements.

SUMMARY OF THE INVENTION

Therefore, the purpose of the present invention is to provide an objectlens unit for optical pickup devices that can be manufactured relativelyeasily and that enable to carry out the relative positioning between thedifferent optical elements and the alignment relative to the holderaccurately and efficiently during the manufacturing process. Inaddition, it is a purpose of the present invention to provide an opticalpickup device using the same.

To solve the above problems, the object lens unit for optical pickupsaccording to the present invention is provided with a first opticalelement placed facing the optical information recording medium, a secondoptical element placed on the light source side of the first opticalelement, and a frame that has a first and a second reference surfacethat are in contact respectively with the first and the second opticalelements to position the two optical elements in the optical axisdirection, wherein the frame fixes the first and the second opticalelements in an integrated manner, and the first and second opticalelements along with the frame are fixed to other members as a singleoptical component.

In the object lens unit of the invention, because accurately machinedreference surfaces are provided on the frame and because each opticalelement is directly in contact with each of the reference surfaces to befixed, accurate relative positioning between the elements is possible.At the same time, because each optical elements is secured to the framein an integrated manner, it has a specific accuracy and can be handledas an independent optical component. Furthermore, by having aconstruction of three components, being the two optical elements and aframe, the shape and structure of each part is simplified and theirmanufacturing becomes easier. Further, though there are cases in whicheach reference surface is at an angle to the optical axis, a desiredprecise tilt angle can be ensured by this reference surface.

In order to solve the above problem, the optical pickup device accordingto the present invention is provided with the above object lens unit, aholder that supports the object lens unit and is shifted along with theobject lens unit, and a fixing means that fixes the object lens unit tothe holder, and has a feature that a light spot is formed on therecording surface of the optical information recording medium by saidobject lens unit.

The object lens unit can be handled as an independently integrallyshaped optical component even though it has high condensingcharacteristics due to two optical elements. Therefore, since therelative positions of the different optical elements inside the objectlens unit have been adjusted at a high degree of accuracy, in the abovefixing means, there is no need to give consideration for the accuracy ofthe optical characteristics of each optical element inside the objectlens unit, and it is sufficient if it can merely improve the accuracy ofmounting the object lens unit to the holder. In the optical pickupdevice of the present invention, since an object lens unit of the abovetype is incorporated, it is possible to obtain an inexpensive unit at ahigh accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of the object lens unit of thefirst preferred embodiment.

FIG. 2 is a cross-sectional side view of the object lens unit of thesecond preferred embodiment.

FIGS. 3(a) to 3(c) are the plan view and cross-sectional side viewdiagrams of the object lens unit of the third preferred embodiment.

FIG. 4 is a cross-sectional side view of the object lens unit of amodified example of the third preferred embodiment.

FIG. 5 is a plan view illustrating the structure of the optical pickupdevice of the fourth preferred embodiment.

FIGS. 6(a) and 6(b) are diagrams illustrating the mounting of the objectlens unit in the optical pickup device.

FIG. 7 is a perspective view showing the object lens unit mounted on theholder.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the invention to overcome the above problemswill now be explained.

Further, as another embodiment of the present invention, in an objectlens unit, the frame is provided with the first and the second fittingportions in which first and the second optical elements are respectivelyfitted and that is used to determine the relative positions between theoptical elements in a direction perpendicular to the optical axis. Inthis case, because the relative position is determined between theoptical elements concerning the direction perpendicular to the opticalaxis by the first and the second fitting portions, it is possible toadjust the optical performance characteristics to a still higher degreeof accuracy of the optical unit combining the two optical elements.

In order to solve the above problem, another object lens unit of theoptical pickup according to the present invention comprises a firstoptical element placed facing the optical information recording medium,a second optical element placed on the light source side of the firstoptical element, a frame that has not only a first and a secondreference surface both of which are in respective contact with the firstand the second optical elements, and that position these elements in theoptical axis direction to fix the first and the second optical elementsas an integrated manner, but also has a third reference surface that isused to position the first and second reference surfaces relative toother members. The frame, further, has a first fitting portion and asecond fitting portion in which the first and the second opticalelements are respectively fitted wherein the fitting portions are usedto determine the relative position between the optical elements in thedirection perpendicular to the optical axis, and a third fitting portionthat is used to determine the relative position of the first and secondfitting portions relative to the other members in a directionperpendicular to the optical axis.

In the object lens, condensing control is achieved mainly by two opticalelements, and in addition, since the relative positioning of each of theoptical elements related to the optical axis is done certainly using thefirst and the second reference surfaces provided in the frame of theobject lens unit, and also, since determining the relative positionsbetween the optical elements related to the direction perpendicular tothe optical axis is done by the first and the second fitting portions,it is possible to adjust the optical performance characteristics of theoptical unit combining the two optical elements at high accuracy. Inaddition, apart from the reference surfaces for those optical elements,since the frame has a third reference surface and a third fittingportion, it is possible to carry out certainly the mounting of the frameto the holder which is a separate member from the first fitting portion,from the second fitting portion, and from the object lens unit. It isalso possible to carry out alignment of each of the optical elements inthe direction perpendicular to the optical axis and positioning in thedirection of the optical axis. Furthermore, by having a constructionwith the three components of the two optical elements and a frame, theshape and structure of each part is simplified, and overallmanufacturing of the device becomes easier. Normally, in theconventional case of directly mounting the two optical elements to thelens holder, for example, since the optical axis adjustment and adhesionis done while inverting the lens holder in the up and down direction,and since the shape of the lens holder is complicated as well as large,the manufacturing equipment for its assembly also becomes large. Incontrast with this, in the case of mounting the two optical elements tothe frame, the manufacturing equipment can be made smaller to carry outassembly. Because of this, the assembly and adjustment become easier.

Further, in another embodiment of the present invention, the frame isprovided with a specific clearance perpendicular to the optical axis inat least one of the first and second fitting portions. In this case, inat least one of the first and the second fitting portions, movementbecomes possible in a direction perpendicular to the optical axis duringalignment due to that clearance, and fine adjustment becomes possible toadjust the center positions of the two optical elements.

Further, in another embodiment of the present invention, in the objectlens unit, the first optical element, the second optical element, andthe frame are each molded plastic components. In this case, not only themanufacturing of each optical element is simplified and the degree offreedom for molding increases, but also the manufacturing of the objectlens becomes easier and the cost of overall manufacturing of the unitbecomes cheaper. Also, here, the term “molded plastic component” is notlimited to components made of pure plastic but also includes componentsmade of plastic to which glass has been added. For example, the casewhen the frame is a molded component made of plastic, and the case whenit is a molded component made of plastic to which 5 to 45% glass hasbeen added, are both included in the expression “molded plasticcomponent”. One of the desired effects of adding glass to plastic is anincrease in strength of the frame.

Further, in another embodiment of the present invention, a secondoptical element is formed to have an orbicular zone structure in whichit is divided into a plurality of orbicular zones on at least one of theoptical surfaces and the neighboring orbicular zones cause a specificoptical path difference to the incident light. In this case, using theaberration correction function of the orbicular structure, it becomespossible to allow the object lens unit to be compatible with severaltypes of optical information recording media, to correct coloraberration, and to correct temperature characteristic aberration.

Further, in another embodiment of the present invention, the aboveorbicular zone structure is a diffraction structure and the secondoptical element is a diffraction lens. In this case, a plurality oflasers used in the optical pickup device are subjected to thediffraction phenomenon and form diffracted light according to thewavelength. It is possible to condense diffracted light of differentorders corresponding to each wavelength as the information reading lightor the information recording light using the differences of theinterference conditions.

Further, in another embodiment of the present invention, the frame isprovided with an aperture. In this case, unwanted light can be blockedor the amount of light can be adjusted.

Still further, in another specific embodiment according to the presentinvention, the above aperture is positioned between said first opticalelement and said second optical element. In this case, there is no needto provide another aperture in the optical pickup device installed withthis object lens unit, and hence it can be downsized.

Yet further, in another specific embodiment according to the presentinvention, the above aperture is positioned on the light source side. Inthis case, unwanted light that becomes cause of visual noise can beblocked before it enters the object lens unit, and it is also possibleto adjust the amount of laser light.

Further, in another embodiment of the present invention, the frame isprovided with a reflection prevention means at least on the insidefacing the optical surface of the optical element. In this case, it ispossible to prevent generation of visual ghost and to increase thetracking accuracy.

Further, in another embodiment of the present invention, the frame isprovided with an adhesive collecting section formed in conjunction withan optical element and having a cross-sectional V-shape or otherappropriate shape in at least one of the first and second fittingportions. In this case, it is possible to ensure strong adhesion and toprevent the adhesive from protruding beyond the periphery during orafter adhering.

Further, in another embodiment of the present invention, the frame has atube-shaped form, and also on the two ends of which said first referencesurface and first fitting portion as well as said second referencesurface and second fitting portion are respectively provided, and also adiscontinuous adhesion section is formed along the outer periphery of atleast one of the first and the second fitting portions. In this case,air is not kept sealed in the space surrounded by the first opticalelement, the second optical element, and the frame, which preventscondensation.

Further, in another embodiment of the present invention, the frame has atube-shaped form, and on the two ends of which said first referencesurface and first fitting portion as well as said second referencesurface and second fitting portion are respectively provided, andfurther a ventilation hole connecting the interior of the frame with theexterior is provided at a specific location on the side surface betweenthe first and the second reference surfaces. In this case, air is notkept sealed in the space surrounded by the first optical element, thesecond optical element and the frame, which prevents condensation.

Further, in another embodiment of the present invention, the frame has atube-shaped form, and on the two ends of which said first referencesurface and first fitting portion as well as said second referencesurface and second fitting portion are respectively provided, and acut-out part connecting the interior of the frame with its exterior isprovided on at least one of a combination of the first fitting portionand the first reference surface in contact with it and a combination ofthe second fitting portion and the second reference surface in contactwith it. In this case also, air is not kept sealed in the spacesurrounded by the first optical element, the second optical element andthe frame, which prevents condensation.

Other preferred embodiments of the invention to solve the above problemswill now be explained.

THE FIRST PREFERRED EMBODIMENT

FIG. 1 is a cross-sectional side view of the object lens unit 50 of thefirst preferred embodiment, and this object lens unit 50 is configuredso that it is compatible with three types of optical disks (high densityDVD, DVD, and CD) of different standards (such as recording density) andhas been configured so that it can carry out recording/reproduction ofinformation in these optical disks. Further, this object lens unit 50 ispositioned facing the optical disks D1, D2, and D3 which are opticalinformation recording media corresponding to high density DVD, DVD, andCD, and has the first optical element B which is the first opticalelement that forms the focus spot on the optical disk D1 by condensingthe light from the light source, the second optical element A which isthe second optical element that forms diffracted light, and thetube-shaped frame 3 which fixes the first optical element B and thesecond optical element A in an integrated manner. Here, the firstoptical element B, the second optical element A, and the frame 3 areformed either of plastic or of plastic to which 5 to 45% of glass fiberhas been added.

The first optical element B is a biconvex lens with the side towards theoptical disks D1, D2, and D3 being relatively flat and the side towardsthe second optical element A is an aspheric surface protruding outwardsubstantially, and condenses the diffracted light of the diffractionorder corresponding to the different wavelengths from the second opticalelement A onto specific locations of each of the optical disks D1, D2,and D3.

The second optical element A has an orbicular zone structure in whichthe element is separated into a plurality of orbicular zones, and byhaving a diffraction structure formed so that a specific optical pathdifference is generated by the neighboring orbicular zones for theincident light beam, a diffracted light is formed corresponding to thelaser light having different wavelengths corresponding to differentoptical disks. At this time, by using the aberration correction functionof the orbicular zone structure, it becomes possible to allow the objectlens unit 50 to be compatible with several types of optical disks D1,D2, and D3, to correct color aberration, and to correct the temperaturecharacteristics aberration. In the case of the present preferredembodiment, by condensing diffracted light of different orders ofdiffraction for each wavelength of laser light by the combination of thefirst optical element B and second optical element A, it becomespossible to carry out imaging with higher accuracy than specific one forall the laser lights, and it becomes possible to use that laser light asthe information reading light or the information recording light foreach of the optical disks D1, D2, and D3.

Here, the degree of condensing by the second optical element A is weakcompared to the degree of condensing by the first optical element B.More concretely, when the paraxial power of the first optical element Bis taken as P1 [m⁻¹] and the paraxial power of the second opticalelement A is taken as P2 [m⁻¹], their values are such that therelationship −0.2≦P2/P1≦0.2 is satisfied. In this case, since theparaxial power of the second optical element A becomes small, thecurvature of the optical surface of the second optical element A becomessmall. As a result, since the effect of the shadows due to the stepparts of the orbicular zone structure can be lessened, the reduction inthe efficiency of use of the laser can be prevented. Further, byreducing the curvature of the optical surface on which the orbicularzone structure of the second optical element A is formed and by makingthe curvature of the first optical element B large, it is possible tomaintain a large pitch of the orbicular zone structure and to suppress,to a low value, the reduction in the efficiency of use of the laser dueto the manufacturing tolerances of the shape of the orbicular zonestructure. In addition, by making the paraxial power of the secondoptical element A small, a large distance (working distance) can beobtained between the object lens 50 and each the optical disks D1, D2,and D3.

The frame 3 has a cylindrical overall external shape, the two ends ofwhich are provided with the cylindrical first fitting portion 4 andsecond fitting portion 5, ring-shaped first reference surface 6 andsecond reference surface 7, ring-shaped aperture 10, ring-shaped thirdreference surface 11, and the third fitting portion 12. The firstfitting portion 4 and the second fitting portion 5 determine therelative positions in the direction of the optical axis of the firstoptical element B and second optical element A. At the time of mountingthese, the first reference surface 6 and second reference surface 7respectively connect the first and second fitting portions, and becomethe reference for aligning the first optical element B and secondoptical element A in the direction of the optical axis and relating tothe inclination against the axis. The second fitting portion 5 isprovided with a clearance 8. This clearance 8 is intended for carryingout fine adjustment of the center position of the second optical elementA against the first optical element B at the time of carrying outalignment of the second optical element A. A chamfer-shaped adhesivecollecting section 9 is provided at the end parts of the first andsecond fitting portions 4 and 5. A V-shaped groove is formed by thecombination of this adhesive collecting section 9 and the outerperipheries of the flange parts 1 a and 2 a of the first optical elementB and the second optical element A, and this groove prevents theadhesive from protruding to the surroundings during or after adheringthese two optical elements A and B. Here, flange parts 1 a and 2 a areretaining portions provided on the periphery of optical effectivesurfaces of the first optical element B and the second optical element Arespectively. They are fixed by adhering while they are directly incontact with the ring-shaped first reference surface 6 and secondreference surface 7, and further, they are directly in contact with thecylindrical first fitting portion 4 and second fitting portion 5, orfacing these fittings with clearances. Thereby, the flanges fill therole of maintaining constant relative positions of optical effectivesurfaces of the optical elements against the frame. The aperture 10placed between the first and second fitting portions 4 and 5 and on theinner wall of the frame 3 carries out the cutting off of unwanted lightor the adjustment of the amount of light at the time of using the objectlens 50. The third reference surface 11 becomes the reference related tothe direction of the optical axis or the inclination, at the time ofmounting the object lens unit to the holder which is a component of theoptical pickup device, and the third fitting portion 12 becomes thereference related to a direction perpendicular to the optical axis(details will be given later).

The manufacturing process of the object lens 50 according to the presentpreferred embodiment is described in the following. To begin with, thefirst optical element B is fixed to the frame 3. The flange surface ofthe first optical element B is made to come into contact with the firstreference surface 6, and adhesive is inserted into the adhesivecollecting section 9. At this time, the positioning in a directionperpendicular to the optical axis is carried out because the firstfitting portion 4 and the peripheral part of the flange part 1 a of thefirst optical element B come into contact with each other. Because ofthis, first optical element B gets fixed at the specific position. Atthis time, since excessive adhesive gets collected in the adhesivecollecting section 9, the adhesive from protruding over the peripherycan be prevented either during or after adhering.

Next, the second optical element A is fixed on the side opposite to thatof the first optical element B mounted on the frame 3. The positioningwith the first optical element B is done by making the flange surface ofthe second optical element A come into contact with the second referencesurface 7. At this time, the internal diameter of the second fittingportion 5 has been designed to be slightly larger than the outerdiameter of the second optical element A, and this difference becomesthe clearance 8 (for example, approx. 50 μm). Because of this, thesecond optical element A becomes movable in a direction perpendicular tothe optical axis, and it is possible to carry out the relativepositioning related to the direction perpendicular to the optical axiswhile watching the opposing first optical element B. Further, at thistime, in order to carry out the positioning definitely, it is alsopossible that markers that become the reference for positioning areformed at the center positions, etc., of the optical surfaces of each ofthe first optical element B and the second optical element A. Themarkers are desirably formed on the optical surfaces of the firstoptical element B and of the second optical element A that come close toeach other, so that they can be seen easily when watching them under amicroscope.

After carrying out the positioning, adhesive is inserted into theadhesive collecting section 9. Because of this, the second opticalelement A gets fixed in the aligned condition. At this time, sinceexcessive adhesive gets collected in the adhesive collecting section 9,the adhesive from protruding over the periphery can be prevented eitherduring or after adhering. Further, the aperture 10 also prevents theadhesive from dripping when it oozes into the interior of the frame 3.

The object lens unit 50 is manufactured in the above manner. As has beendescribed above, the object lens 50 has a three-component constructionusing the frame 3 in addition to the first optical element B and thesecond optical element A. Because of this, it is possible to carry outbeforehand the relative positioning of the first optical element B andthe second optical element A concerning the direction perpendicular tothe optical axis with high precision.

Further, regarding the sequence of mounting, in the present description,the mounting of the first optical element B has been made first and themounting of the second optical element A has been made next. Althoughthis is due to convenience of designing because it is easier to observefrom the side of the second optical element A with the smallerrefractive power at the time of positioning, it is also possible tocarry out the mounting by changing the order of assembling. Whenchanging the order of mounting, the clearance is provided on the side ofthe first fitting portion 4. In addition, any one of the two fixingmeans can also not use an adhesive but can also be a squeeze fittingutilizing the difference in contraction upon heating. Further, otherfixing means can be used, for example, the two optical elements A and Bcan be fixed to the frame 3 by laser welding. Further, the clearanceneed not be provided only on side of the second optical element A, butit can be provided both on the side of the second optical element A andon the side of the first optical element B.

Further, in any of the cases described above, regarding the firstoptical element B and the second optical element A, if the first opticalelement B and the second optical element A are manufactured so that noburrs are present on the flange surfaces that come into contactrespectively with the first reference surface 6 and the second referencesurface 7 (In FIG. 1, the left side surface of flange 1 a of the firstoptical element B and the right side surface of flange 2 a of the secondoptical element A), the lowering of the mounting accuracy due to burrscan be avoided even if burrs are present on the sides opposite to thesesurfaces.

Further, at the time of bonding, if the frame 3 has a tube-shaped form,although the first fitting portion 4 and the second fitting portion 5are ring-shaped, the bonded part fixed by the supply of the adhesive canbe present not over the entire fitting portions 4 and 5 but can bepresent discontinuously if the bonding strength is sufficient. By makingthe bonding-section discontinuous and by providing non-bonded locationsin the above manner, the ventilation can be improved.

Further, the frame 3 has an anti-reflection coating which is theanti-reflection means on the inner surface side of the tube-shaped form.The anti-reflection coating is formed, for example, by coating the innersurface with the black-colored paint. Due to this, the generation ofghosts and can be prevented to increase the accuracy of tracking.Further, instead of forming an anti-reflection coating on the insidesurface of the frame 3, the frame using a material with light absorptioncharacteristics can be formed.

Further, it is also possible that the frame 3 is provided with aprotector on the side of the optical disks D1, D2, and D3 for preventingcontact between the lens and each of the optical disks D1, D2, and D3.

The laser light that impinges on the second optical element A from theside of the light source (left side in FIG. 1) and that has differentwavelengths corresponding to the optical disks D1, D2, and D3 isconverted into a diffracted light by the second optical element A. Thediffracted light of different order of diffraction depending on thewavelength is condensed by the first optical element B and a spot of itis formed on each of information recording surfaces M1, M2, and M3 ofthe optical disks D1, D2, and D3. The laser light reflected on theoptical disks D1, D2, and D3 is collected via the first optical elementB and the second optical element A, and it is guided to the sensor whichis not shown in the figure. Because of the presence of such an objectlens unit 50, three types of laser light can be used as the informationreading light or the information recording light for the optical disksD1, D2, and D3.

Further, here, by providing a frame 3 with an aperture 10 between thefirst optical element B and the second optical element A, it not onlybecomes possible to cut off unwanted light that becomes the cause ofnoise or to adjust the amount of light, but also it becomes unnecessaryto provide a new aperture in the optical pickup device installed withthe object lens unit 50 whereby the unit can be made compact.

THE SECOND PREFERRED EMBODIMENT

In the first preferred embodiment, as is shown in FIG. 1, although theaperture 10 has been positioned between the first optical element B andthe second optical element A, the aperture 10 can also be positioned onthe light source side of the first optical element B and the secondoptical element A. FIG. 2 is a side cross-sectional view of the objectlens unit 150 of the second preferred embodiment in which the aperture110 is present on the light source side. In the present preferredembodiment, since the object lens unit 150 has a structure identical tothat of the object lens unit 50 of the first preferred embodiment,except for the structure of the frame 103, its explanation will beomitted here. In the frame 103 according to the present preferredembodiment, the aperture 110 is positioned, in the figure, at the leftend (light source side) of the frame 103. Further, because of this,unlike in the first preferred embodiment, the second reference surface107 comes into contact with the light source side of the second opticalelement A (at the left side in the figure). In addition, because ofthis, in the manufacturing process, both the first optical element B andsecond optical element A are formed by dropping them in from the opticaldisk side.

In this preferred embodiment, because of placing the aperture 110 on thelight source side of the first optical element B and the second opticalelement A, the unwanted light that becomes the cause of noise can be cutoff before it enters the object lens unit 150, and it also becomespossible to adjust the amount of laser light.

THE THIRD PREFERRED EMBODIMENT

FIG. 3 is a view for explaining the object lens unit 250 according tothe third preferred embodiment of the present invention, and this objectlens unit 250 has the cutout CT for ventilation. FIG. 3(a) is the planview of seeing the frame 203 from the side of the first optical elementB, and FIG. 3(b) is the cross-sectional view on the S-S line, and FIG.3(c) is a cross-sectional side view showing the state of mounting thefirst optical element B and second optical element A on the frame.Further, all descriptions will be omitted in this preferred embodimentof items other than that is explained afresh because they are identicalto the other preferred embodiments.

In this preferred embodiment, the first fitting portion 4 that ispresent on the inside of the frame 203 being approximately ring-shapedhas additionally a single or a plurality of cutouts CT with a specificspacing (in specific terms, at three locations as is shown in FIG.3(a)). Here, the first fitting portion 4 has an approximate ring shapecomprising a discontinuous ring-shaped part and the plane sections 4 a,4 b, and 4 c. The first optical element B is positioned in a directionperpendicular to the optical axis by these flat sections 4 a, 4 b, and 4c. Further, the frame 203 has a plurality of contact sections 6 a, 6 b,and 6 c that carry out the role of the first reference surface 6 as thesupporting section of the first optical element B. Each of these contactsections 6 a, 6 b, and 6 c is a plane perpendicular to the optical axis,and these are placed at approximately equal intervals along theperipheral directions. As is shown in FIG. 3(c), the first opticalelement B is aligned by being supported by each of the contact sections6 a, 6 b, and 6 c. While the flange of the first optical element B isfixed to the frame 203 via the contact section 6 a, an air vent hole isformed by the cutout CT between the first optical element B and theframe 203. Because of this, air is not kept sealed in the interior ofthe object lens unit 250, and hence, it is difficult for condensation tooccur. Furthermore, at the time of bonding the first optical element B,by making the bonding locations discontinuous and not over the entireadhesive collecting section 9 thereby providing non-bonded portions, theventilation can be ensured.

Further, as can be seen in FIG. 3(b), even on the side of the secondoptical element A, it is also possible to provide a contact section 7 aand the like. Because of this, at the time of supporting the secondoptical element A by the contact section and bonding it, by making thebonding locations discontinuous and not over the entire adhesivecollecting section 9 thereby providing non-bonded portions, theventilation can be ensured. In addition, it is also possible to providea cutout for further ventilation even on the second fitting portion 5.

Further, ventilation of air to the outside of the frame 203 can beachieved by measures other than the above cutouts CT for ventilation.Although the object lens unit 350 of the modified example shown in FIG.4 has an overall construction that is identical to that of the objectlens unit 50 shown in FIG. 1, ventilation of air to the outside of theframe 303 has been achieved by providing one or more ventilation holesAH (at one location in FIG. 4) on it.

Coming back to FIG. 3(a), in the present preferred embodiment, inaddition, a cutout NT has been formed on the circumferential edge of theframe 203. The cutout NT is used, for example, as a reference mark thatbecomes the index of the direction of the astigmatism of the object lensunit 250. By fixing it to the holder in the condition in which thecutout NT is oriented in a specific direction, it becomes possible todesign the optical pickup device to which consideration of the directionof astigmatism is given.

THE FOURTH PREFERRED EMBODIMENT

FIG. 5 is a plan view explaining the structure of the optical pickupdevice 100 of the fourth preferred embodiment. The optical pickup device100 according to the present preferred embodiment is compatible withthree types of optical disks conforming to different standards (highdensity DVD, DVD, and CD), and has been constructed so as to makeinformation recording and reproduction possible with these opticaldisks.

The optical pickup device 100 is provided with semiconductor lasers LD1,LD2, and LD3, a beam shaper SH1, a collimator CL, beam splitters BS1,BS2, BS3, BS4, and BS5, cylindrical lenses L11, L21, and L31, concavelenses L12, L22, and L32, photo detectors PD1, PD2, and PD3, a objectlens unit 50, a holder 60, and a 2-dimensional actuator 70. The objectlens unit 50 is mounted to the optical pickup device 100 via the holder60 in a movable manner. In the description of this preferred embodimentgiven here, although the object lens unit 50 of the first preferredembodiment has been used as the object lens unit, it can be alsoreplaced by the object lens units 150, 250 and 350 of other preferredembodiments.

In this optical pickup device 100, regarding high density DVD, DVD, andCD, the laser light L1 of a wavelength of 405 nm emitted from thesemiconductor laser LD1, the laser light L2 of a wavelength of 650 nmemitted from the semiconductor laser LD2, the laser light L3 of awavelength of 780 nm emitted from the semiconductor laser LD3 are usedrespectively for reading out the information from the informationrecording surfaces of the optical disks D1, D2, and D3 for high densityDVD, DVD, and CD.

When carrying out data recording or reproduction by the optical disk D1for high density DVD, the laser light L1 with a wavelength of 405 nmemitted from the semiconductor laser LD1 is shaped by passing throughthe beam shaper SH1, passes through the beam splitter BS1, getsconverted into a parallel beam by the collimator CL, passes through thebeam splitters BS4 and BS5, and then enters the object lens unit 50.Next, by the object lens unit 50, the laser light L1 is condensed on theinformation recording surface M1 of the optical disk D1 having atransparent protective substrate.

The laser light L1 modulated and reflected by the information pit on theinformation recording surface M1 again passes through the object lensunit 50, the beam splitters BS5 and BS4, and the collimator CL, and getsreflected by the beam splitter BS1, has astigmatism applied to it by thecylindrical lens L11, passes through the concave lens L12, and entersthe photo detector PD1. The read out signal of the information recordedin the optical disk D1 is obtained using the signal output by the photodetector PD1.

When carrying out data recording or reproduction by the optical disk D2for DVD, the laser light L2 with a wavelength of 650 nm emitted from thesemiconductor laser LD2 passes through the beam splitter BS2, getsreflected by the beam splitter BS4, passes through the beam splitterBS5, and then enters the object lens unit 50. Next, by the object lensunit 50, the laser light L2 is condensed on the information recordingsurface M2 of the optical disk D2 having a transparent protectivesubstrate.

The laser light L2 modulated and reflected by the information pit on theinformation recording surface M2 again passes through the object lensunit 50 and the beam splitter BS5, gets reflected by the beam splittersBS4 and BS2, has astigmatism applied to it by the cylindrical lens L21,passes through the concave lens L22, and enters the photo detector PD2.The read out signal of the information recording on the optical disk D2is obtained using the signal output by the photo detector PD2.

When carrying out data recording or reproduction by the optical disk D3for CD, the laser light L3 with a wavelength of 780 nm emitted from thesemiconductor laser LD3 passes through the beam splitter BS3, getsreflected by the beam splitter BS5, and then enters the object lens unit50. Next, by the object lens unit 50, the laser light L3 is condensed onthe information recording surface M3 of the optical disk D3 having atransparent protective substrate.

The laser light L3 modulated and reflected by the information pit on theinformation recording surface M3 again passes through the object lensunit 50, gets reflected by the beam splitters BS5 and BS3, hasastigmatism applied to it by the cylindrical lens L31, passes throughthe concave lens L32, and enters the photo detector PD3. The read outsignal of the information recorded on the optical disk D3 is obtainedusing the signal output by the photo detector PD3.

At the time of recording or reproducing high density DVD, DVD, and CD,focusing detection and tracking detection are carried out by detectingchanges of light quantity from changes in the shape and changes in theposition of the spot on the photo detectors PD1, PD2, and PD3. Based onthe result of this detection, the 2-dimensional actuator 70 moves theholder 60 along with the object lens unit 50 so that the laser light L1,L2, and L3 from the semiconductor lasers LD1, LD2, and LD3 are allowedto form images on the information recording surfaces M1, M2, and M3 ofthe optical disks D1, D2, and D3, and also, moves the holder 60 alongwith the object lens unit 50 so that the laser light L1, L2, and L3 fromthe semiconductor lasers LD1, LD2, and LD3 are allowed to form images ona specific track.

Further, aberration correction of the first optical element B has beenconducted for the transparent protective substrate (thickness of 0.1 mm)of the optical disk D1 for high density DVD, and the designed wavelengthis 405 nm, focal length is 2.2 mm, and the numerical aperture number onthe recording media side is 0.85.

When the first optical element B is used for recording or reproducingdata with a DVD (wavelength: 650 nm, numerical aperture number on therecording media side: 0.65, thickness of transparent protectivesubstrate: 0.6 mm) or a CD (wavelength: 780 nm, numerical aperturenumber on the recording media side: 0.50, thickness of transparentprotective substrate: 1.2 mm), the spherical aberration due to thedifference in the thickness of the transparent protective substratechanges in the direction of overcorrection. Even if the third orderspherical aberration component of the spherical aberration changingtowards overcorrection is removed by allowing the radiating light fluxto enter the first optical element B, higher order spherical aberrationcomponents still remain, and in that condition information recording andreproduction with DVD and CD cannot be carried out.

Further, the semiconductor laser LD1 which is the light source for highdensity DVD is a blue-violet semiconductor laser and it is said that itswavelength changes by about 1 nm due to mode hopping. When thewavelength of the incident light becomes 406 nm which is longer by 1 nmthan the designed wavelength of the first optical element B, the bestimaging position evaluated by wave front aberration changes by 0.49 μm,due to which a defocusing component is added and the wave frontaberration gets deteriorated to 0.162 arms. Therefore, stableinformation recording and reproduction cannot be made since thecondensing performance gets deteriorated substantially for an opticaldisk D1 of high density DVD during mode hopping when the first opticalelement B is used alone.

In addition, if it is assumed that the change in the refractive indexdue to temperature raise of the first optical element B is −9.0×10⁻⁵/°C. and that the rate of change of the wavelength of the blue-violetsemiconductor laser due to temperature raise is 0.05 nm/° C., thespherical aberration of the first optical element B changes towardsovercorrection due to a temperature rise of 30° C., and the wave frontaberration during recording and reproduction of a high density DVDdeteriorates to 0.145 λrms. Therefore, stable information recording andreproduction cannot be made since the condensing performance getsdeteriorated substantially for an optical disk of high density DVDduring a temperature rise when the first optical element B is usedalone.

In view of this, by using the diffraction effect of the diffractionstructure formed in the second optical element A, the object lens unit50 of the present preferred embodiment compensates for changes in thespherical aberration due to difference in the thickness of thetransparent protective substrate, changes in the best imaging positioncaused by mode hopping in the blue-violet semiconductor laser LD2, andthe changes in the spherical aberration caused by changes in therefractive index associated with changes in the temperature.

Further, by providing an appropriate ring-shaped diffraction structureon the optical surface of the second optical element A, the object lensunit 50 according to the present preferred embodiment can be designed sothat information recording and reproducing is possible in all cases ifthe light for information recording and reproduction used is, forexample, the 6th order diffraction light generated by the diffractionstructure of the second optical element A for a high density DVD, the4th order diffraction light generated by the diffraction structure ofthe second optical element A for a standard DVD, the 3rd orderdiffraction light generated by the diffraction structure of the secondoptical element A for a CD.

When carrying out information recording and reproduction such as theabove, for example, it is necessary to suppress the comatic aberrationin a DVD to 0.02 λ or less. In order to satisfy this, it is necessary toadjust the relative positions of the first optical element B and secondoptical element A in a direction perpendicular to the optical axis witha high accuracy of within a few μm. Therefore, for example, it isimportant to provide the clearance 8 in each of the fitting portions 4and 5 and to carry out bonding while making adjustment.

FIG. 6(a) is a plan view for explaining the mounting of the object lensunit 50 according to the present preferred embodiment in an opticalpickup device 100, FIG. 6(b) is its cross-sectional view at A-A, whichcorresponds with FIG. 5. By mounting the object lens unit 50, themanufacturing process of which was described in the first preferredembodiment, to the holder 60 which is a part of the optical pickupdevice 100, the object lens unit 50 can be used as a component member ofthe optical pickup device 100.

The holder 60 is provided with the unit mounting section 61 for theobject lens unit 50. The object lens unit 50 takes the third referencesurface 11 provided on the frame 3 as the reference for the mountingposition and the third reference surface 11 is supported by the contactsection 62 provided on the unit mounting section 61. In addition, thethird fitting portion 12 mates with the side surface section 63 of theunit mounting section 61. Because of this, the object lens unit 50 isinstalled at a specific location of the holder 60, and the object lens50 and the holder 60 are fixed by injecting adhesive GL from theperiphery of the unit mounting section 61 (at four locations in FIG.6(a)). Further, because a clearance is formed between the third fittingportion 12 and the side surface section 63, it can be considered thatadjustment for relative positioning of the object lens unit 50 can becarried out with respect to other members in a direction perpendicularto the optical axis. As a modified example of the holder 60 shown in thefigure, in order to prevent adhesive GL from overflowing at the time ofadhering, a V-shaped groove that is formed as the adhesive collectingsection can be provided by the conjunction between the frame 3 and theunit mounting section 61.

Here, regarding the first optical element B and the second opticalelement A, since the alignment operation has already been made duringthe process of manufacturing the object lens unit 50, there is no needto carry out the alignment operation again at the time of mounting tothe holder 60. In addition, because the frame 3 of the object lens unit50 is provided with the third reference surface 11, it is not onlypossible to carry out alignment definitely during mounting to the holder60, but also it is possible to carry out relative positioning of theobject lens unit 50 relative to other members in a directionperpendicular to the optical axis using the third fitting portion 12.

As is evident from the above, by supporting the object lens unit 50 bythe holder 60, and by controlling the drive of the entire holder 60using the two-dimensional actuator 70 provided in the optical pickupdevice 100, the displacement of the object lens unit 50 is carried outin two dimensions.

FIG. 7, in the present preferred embodiment, is a perspective viewshowing one form in which the object lens unit 50 is mounted to theholder 60 inside the optical pickup device 100. The area T correspondsto FIG. 6(a). The drive control of the holder 60 to which the objectlens unit 50 has been mounted is carried out by the two-dimensionalactuator 70 for focusing and tracking. In addition, the entire mechanismis supported and the object lens unit is connected and fixed to the bodyof the optical pickup device 100 by the base plate 80.

The two-dimensional actuator 70 is provided with a focusing coil 71, apair of magnets 72 and 73, yokes 74 and 75 that support the pair ofmagnets 72 and 73, upper connecting sections 76 a and 76 b, a lowerconnection section (not shown in the figure because it is hidden by theholder 60 and the base plate 80), upper elastic supporters 77 a and 77b, lower elastic supporters (not shown in the figure because it ishidden by the holder 60 and the base plate 80), and a tracking coil (notshown in the figure because it is provided inside the two-dimensionalactuator). Further, two lower elastic supporters are present and areplaced right below the upper elastic supporters 77 a and 77 brespectively. Two lower connecting sections are present and are placedright below the upper connecting sections 76 a and 76 b. The upperelastic supporters 77 a and 77 b and the lower elastic supporter areplaced so that they are parallel to the horizontal direction, and oneend of these bodies are fixed after passing through the base plate 80,while the other ends are inserted through the upper connection sections76 a and 76 b and the lower connecting sections that project on bothsides of the holder 60, and are fixed elastically. The holder 60 isdriven in the vertical direction by the focusing coil 71, and driven inthe horizontal direction by the tracking coil which is not shown in thefigure. As is also clear from the figure, the holder 60 has anon-symmetrical structure, and is not suitable for fixing directly thefirst optical element B and the second optical element A with a highaccuracy. In actuality, when directly mounting the first optical elementB and the second optical element A to a conventional type lens holderhaving a structure similar to that of the holder 60, for example, theprocess is that of bonding initially the first optical element B,turning upside down the entire lens holder, and bonding the secondoptical element A while adjusting it in the perpendicular direction tothe optical axis. The lens holder, however, generally has a complicatedshape, and since even its size is considerably large compared to that ofthe frame 3, the manufacturing equipment becomes correspondingly largeand the ease of adjustment work gets lost. On the other hand, when theobject lens unit 50 is used in which the first optical element B and thesecond optical element A have been aligned and fixed beforehand with ahigh accuracy, since all the relative positions between each of thelenses inside the object lens unit 50 have already been determined witha high accuracy, it is possible to carry out easily the mounting of theobject lens system with a relatively high accuracy irrespective of theshape of the holder 60.

1. An object lens unit for an optical pickup comprising: a first opticalelement positioned facing an optical information recording medium; asecond optical element positioned on a light source side of the firstoptical element; and a frame which has a first reference surface incontact with the first optical element to position the first opticalelement in an optical axis direction and a second reference surface incontact with the second optical element to position the second opticalelement in the optical axis direction so that the first optical elementand the second optical element are fixed to be integrated with theframe, wherein the first optical element, the second optical element andthe frame are attached, as a single optical component, to anothermember.
 2. The object lens unit of claim 1, wherein the frame comprisesa first fitting portion and a second fitting portion in which the firstoptical element and the second optical element are respectively fitted,to determine relative positions of the first optical element and thesecond optical element to each other in a perpendicular direction to theoptical axis.
 3. The object lens unit of claim 1, wherein the framecomprises a third reference surface to position the first referencesurface and the second reference surface relative to another member. 4.The object lens unit of claim 2, wherein the frame comprises a thirdfitting portion to determine relative positions of the first fittingportion and the second fitting portion to another member in aperpendicular direction to the optical axis.
 5. The object lens unit ofclaim 1, wherein the first reference surface is perpendicular to theoptical axis.
 6. The object lens unit of claim 1, wherein the secondreference surface is perpendicular to the optical axis.
 7. The objectlens unit of claim 2, wherein the frame has a prescribed clearance in aperpendicular direction to the optical axis in at least one of the firstfitting portion and the second fitting portion.
 8. The object lens unitof claim 1, wherein the first optical element, the second opticalelement and the frame are plastic moldings.
 9. The object lens unit ofclaim 1, wherein the second optical element has an orbicular zonestructure in which the second optical element is divided into aplurality of orbicular zones at least on one optical surface so thatneighboring orbicular zones-cause prescribed optical path differencesfor received incident light.
 10. The object lens unit of claim 9,wherein the orbicular zone structure is a diffraction structure and thesecond optical element is a diffraction lens.
 11. The object lens unitof claim 1, wherein the frame has an aperture.
 12. The object lens unitof claim 11, wherein the aperture is positioned between the firstoptical element and the second optical element.
 13. The object lens unitof claim 11, wherein the aperture is positioned on a light source sideof the second optical element.
 14. The object lens unit of claim 1,wherein the frame has a reflection preventive means at least on an innersurface of the frame.
 15. The object lens unit of claim 2, wherein atleast one of the first fitting portion and the second fitting portionhas an adhesive collecting section forming a groove in conjunction withthe first optical element or the second optical element.
 16. The objectlens unit of claim 2, wherein the frame has a tube-shaped form and hasthe first reference surface and the first fitting portion on one endside and the second reference surface and the second fitting portion onanother end side.
 17. The object lens unit of claim 16, wherein separateadhering sections are formed along at least one of circumferences of thefirst fitting portion and the second fitting portion.
 18. The objectlens unit of claim 16, wherein the frame has a vent hole allowing aninterior and an exterior of the frame to communicate at a prescribedposition between the first reference surface and the second referencesurface.
 19. The object lens unit of claim 16, wherein at least onecombination among a combination of the first fitting portion and thefirst reference surface and a combination of the second fitting portionand the second reference surface has a cutout allowing an interior andan exterior of the frame to communicate.
 20. An optical pickup devicecomprising: the object lens unit of claim 1; a holder supporting theobject lens unit and shifting together with the object lens unit; and afixing means to fix the object lens unit to the holder, wherein theobject forms a light spot on a recording surface of an opticalinformation recording medium.